Work machines include an internal-combustion engine as a power source that generates power for traveling or power for operating an implement, for example. In recent years, for example, as described in Patent Literature 1, there is a work machine in which the internal-combustion engine and a generator motor are combined, and which uses the power generated by the internal-combustion engine as the power of the work machine, and drives the generator motor by the internal-combustion engine to generate electric power.